A. Technical Field
The present invention relates to an absorbent article as favorably used for sanitary materials such as paper diapers (disposable diapers), sanitary napkins, and so-called incontinence pads.
B. Background Art
In recent years, water-absorbent resins are widely utilized as constituents of sanitary materials, such as paper diapers, sanitary napkins, and so-called incontinence pads, for the purpose of allowing the water-absorbent resins to absorb body fluids.
As to the above-mentioned water-absorbent resins, the following materials are, for example, known: crosslinked matters of partially neutralized polyacrylic acids, hydrolysates of starch-acrylonitrile graft polymers, neutralized products of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic ester copolymers, hydrolysates of acrylonitrile copolymers or those of acrylamide copolymers, or crosslinked matters of these copolymers, and crosslinked matters of cationic monomers.
It has so far been said that the above-mentioned water-absorbent resins should be excellent in the following properties: the amount of water as absorbed, the water absorption speed, the gel strength, the suction power to suck up water from a base material containing an aqueous liquid, and so on, upon contact with an aqueous liquid such as a body fluid. In addition, various water-absorbent resins or various absorbent matters or articles using the water-absorbent resins have so far been proposed, wherein the water-absorbent resins have at least two of the above-mentioned properties together and display excellent performances (water absorption properties) when used for sanitary materials such as paper diapers and sanitary napkins.
As to the above-mentioned conventional water-absorbent resins or the above-mentioned conventional absorbent matters or articles using the water-absorbent resins, the following materials are, for example, known: water-absorbent resins having a specific gel capacity, a specific shear elasticity, and an specific extractable-polymer content in combination, water-absorbent resins of which the amount of water as absorbed, the water absorption speed, and the gel strength are specified, and paper diapers or sanitary napkins using these water-absorbent resins, and paper diapers using water-absorbent resins displaying a specific amount of water as absorbed, a specific water absorption speed, and a gel stability, and water-absorbent articles using water-absorbent resins of which the amount of water as absorbed, the suction power, and the amount of water-soluble components are specified, and water-absorbent sanitary articles containing water-absorbent resins of which the amount of water as absorbed, the amount of water as absorbed under a load, and the gel fracture strength are specified (JP-A-63-099861); paper diapers containing water-absorbent resins of which the amount of water as absorbed and the water absorption speed under a load are specified, and water-absorbing agents containing water-absorbent resins of which the amount of water as absorbed under a load and the particle diameter are specified (European Patent No. 339,461); water-absorbing agents containing water-absorbent resins in not smaller than a specific amount wherein the water absorption speed and the amount of water as absorbed under a load in a short period of time are specified with regard to the water-absorbent resins (European Patent No. 443,627); water-absorbent combined materials containing water-absorbent resins in not smaller than a specific amount wherein the shape change and the suction index, both under a load, are specified with regard to the water-absorbent resins (European Patent No. 532,002); absorbent articles using resins of which the pressure absorption index and the 16-hour extractability level are regulated (European Patent No. 615,736).
In recent years, the absorbent articles such as paper diapers and sanitary napkins are advancing in high functionalization and thinning, and the amount of water-absorbent resins as used per sheet of the absorbent articles or the ratio by weight of water-absorbent resins to the total of the water-absorbent resins and fibrous materials tends to increase. In other words, attempts are made to increase the ratio of water-absorbent resins in absorbent matters by lessening fibrous materials with a small bulk density and increasing the water-absorbent resins with excellent water absorbency and a large bulk density, and to thereby thin the sanitary materials without lowering the amount of water as absorbed.
At present, there is not yet any clear answer to problems about what property is most necessary to the above-mentioned water-absorbent resins for increasing the absorption amount of absorbent articles in practical use when producing absorbent articles containing a relatively large amount of water-absorbent resins, in other words, for producing absorbent articles which merely involve leakage as little as possible.
In addition, there is not any clear answer, either, to problems about that how to change properties of water-absorbent resins, as used, for the purpose of increasing or keeping the absorption amount of absorbent articles as final products when changing the resin ratio (the ratio by weight of water-absorbent resins to the total of the water-absorbent resins and fibrous materials) in absorbent matters is preferable in technical and economical respects.
In addition, there are also problems in that: performances of absorbent articles containing a relatively large amount of water-absorbent resins greatly depend on the water-absorbent resins, and the absorption amount of the absorbent articles is easily influenced by property differences of water-absorbent resins, as used, between makers thereof, and by performance variations of water-absorbent resins as used, and further by resin ratio variations and so on.
Thus, there is not yet any solution to problems about what properties of water-absorbent resins should be noted and how such properties should be modified, in response to the change or variation of the resin ratio, for the purpose of allowing absorbent articles such as paper diapers and sanitary napkins to display their objective performances maximally and stably in each state in the case where the resin ratios in absorbent matters are different and for the purpose of preventing differences in performance of the absorbent articles, or otherwise how to change the resin ratio in response to differences or variations of properties of the water-absorbent resins.